Refactor context related elements

[cnouguier]

• The core module must intiailizes the Store if a context is defined in the configuration
• It must relies on the Store to store the current context
• It must provide a composable that helps using the current context
• Remove mixin.base-context.js
• Replace the contextId props with a service prop which can be either a string (id) or an object (name and context)
• Remove unrequired contextId prop when the context should be directly extracted from the store (e.g. map)
• Add a getService(name, 'context') in API when we'd like to explicitely retrieve the current context from the store instead of doing this implicitely when no context is given, when no context is given we should get the global service
• Remove declareService() as it should not be useful anymore with the previous change